1. Field of the Invention
The present invention is directed generally to monitoring operating parameters in a steam turbine and, more specifically, to the monitoring of unshrouded turbine blade vibration.
2. Description of the Prior Art
Turbine blades, because of their complex design, can suffer from vibration at frequencies which correspond to natural frequencies of the blade called modes. Each mode is associated with a different type of vibration such as along the rotational axis of the turbine, perpendicular to the rotational axis of the turbine, etc. To prevent excessive vibration of the blade about its normal position, normal design practice dictates that the blades be constructed such that modes are located between harmonics of the operating frequency of the steam turbine. However, manufacturing tolerances, changes in blade attachment to the rotor, changes in blade geometry due to erosion and changes in the operating frequency of the turbine, among other factors, cause mode frequencies to approach harmonics of the operating frequency. The approach of the modes to the harmonics of the operating frequency may result in physical damage to the steam turbine. When the amplitude of the vibration exceeds a certain level, objectionable stresses are set up in the blade. If the condition is not detected and remedied, the blade may eventually fracture resulting in an extremely costly forced outage of the machinery. Thus, methods and apparatus for detecting such vibrations are necessary to prevent such damage.
An apparatus for detecting the vibration of the blades of a turboengine is disclosed in U.S. Pat. No. 3,467,358 issued Sept. 16, 1969 to Zablotsky et al. In that patent, a pair of non-contact electric pulse transmitters are mounted on stationary parts of the engine near its rotor blades and are displaced in the plane of rotation relative to each other by an amplitude of vibration which indicates either a preset or an excessive value which is to be detected. The transmitters may be positioned on opposite ends of the blade, as shown in FIG. 3 of the Zablotsky Patent. In such an orientation, the angle of displacement between the two transmitters is derived by dividing an indicated amplitude of torsional vibration by the external radius of the rotor. The pulses produced by the transmitters are processed through appropriate circuitry to produce output signals indicative of the detected vibration.
Numerous other systems exist for measuring the vibration of unshrouded turbine blades. However, the need exists for an apparatus and method which are simple to implement and reliable, yet which provide the necessary accuracy to detect dangerous levels of vibration before the turbo machinery is damaged.